Decay effect/texture
Idle Chit-chat

The best way I could think of doing things is to create two separate meshes: the tooth and the decay. Using my method, I created this:



Teeth Creation

I started by modeling a simple tooth. I included the mesh for this image, but deleted it later. Do not worry about it. I also set shading to Smooth in the Tools bar. I then added a material and set the Diffuse color to hex FFF2FE. I did not change anything else in the material.



After that, I added a sphere. I set the shading to smooth. I moved it down to Z: -1.4334 and scaled it so X, Y, and Z were all 1.189. This makes it so just some of the sphere is visible near the gunky area in the reference photo. I added both a Subdivision surface and Displace Object modifier. I set the settings to these:



Important! Before you can set the settings in the Displace modifier, click + New to get a new texture. Name it "Disp."

Next, I went to the Textures tab in the Properties panel. Make sure that  is selected at the top of the tab. Select Displace from the top drop down menu and Disp if it isn't already selected in the next drop down menu. If it is not already set, set the Type: to Clouds.



With the sphere still selected, go to the Materials tab and add a new material. Set the Diffuse color to hex C29F75. Change the Specular intensity: to .1.

Lighting and World setup

Move the lamp infront of the tooth. I used X: 0.0194096, Y: -4.1363487, and Z: 2.4681318. In the Object Data Properties panel tab, set Samples: to 5. Go to the World tab. Set the Horizon Color: to hex 595A66. Check Environment Lighting. Set the Energy: to .4, and in the drop down menu, select Sky Color.

Last Words

I assumed here that you were using BR. If you use Cycles, refer to Duarte Farrajota Ramos's answer. I might tweak the lighting to get a better result. Finally, here is the .blend file: